The use of television is commonplace in the United States and throughout the world. Nearly every home in the United States has at least one television set. Many homes have cable television, which couples a large number of television channels to the home through a single coaxial cable. Other homes and businesses may have satellite receivers that are capable of receiving television signals from a number of satellites in stationary orbit around the earth.
Television signals are defined by the National Television Standards Committee (NTSC). Each television signal comprises a video signal and an audio signal. The NTSC signal, which evolved when only black and white (B/W) television was available, has a bandwidth of approximately 4.8 megahertz (MHz). The NTSC signal is modulated to a predetermined carrier frequency. For example, VHF channel 2 has a carrier frequency of 55.25 MHz. A small spacing in the frequency spectrum between adjacent channels prevents interference between channels. Other transmission systems, such as cable broadcasting, may use different frequencies for the television channels.
When color television was introduced, it was important that the color signals be added in a manner that did not interfere with the normal operation of B/W television signals. This was accomplished by introducing a chrominance signal modulated at a frequency that causes the chrominance signal for each line of the television signal to have an inverted phase with respect to the prior line. There are an odd number of lines in each television frame, with the result being that the chrominance signal for any given line is inverted in alternating frames of the television signal. The phase inversion causes the chrominance signal to cancel out temporally over the time of one frame, and spatially in the vertical axis over the space of two lines. The cancellation prevents the chrominance signal from erroneously being seen by the viewer as part of the luminance signal. This effect, combined with the known persistence of vision in humans causes the chrominance signal to effectively cancel out in a B/W television so that it causes no noticeable interference. The NTSC signal has a modulated chrominance signal that overlaps the luminance signal in a portion of the frequency spectrum where the overlap causes minimal interference.
The frequency spectrum of the NTSC signal is shown in FIG. 1A. As can be seen in FIG. 1A, the video signal comprises a luminance signal 2 and a chrominance signal 4. The luminance signal 2 provides the signal intensity for both B/W and color television signals. The luminance signal 2 has spectral peaks 6 every 15.75 kilohertz (kHz), which corresponds to the horizontal frequency in the television. The amplitude of the luminance spectral peaks 6 decreases up to 4.2 MHz. The video signal is suppressed above 4.2 MHz to permit the insertion of an audio signal 5 in the spectrum for the particular video channel. The audio signal 5 is modulated with a 4.5 MHz carrier.
The chrominance signal 4 is introduced beginning at about 2.1 MHz in the spectrum. The chrominance signal 4 has chrominance spectral peaks 8, which are also spaced 15.75 kHz apart in the frequency spectrum. The chrominance signal is modulated at a frequency of 3.579545 MHz (an odd multiple of half the line scan frequency) to cause the chrominance signal peaks to interlace with the luminance peaks, as shown in FIG. 1B, which illustrates a magnified portion of the spectrum of FIG. 1A.
As seen in FIG. 1B, the luminance spectral peaks 6 and the chrominance spectral peaks 8 are spaced apart by 7.875 kHz. Although FIG. 1B, shows the frequency spectrum with no overlap, there is some degree of overlap in these signals due to the non-periodicity of the signals with respect to the line scan frequency.
The selected carrier frequency and alternating phases cause the additional luminance signal to cancel out temporally and spatially in the same manner as the chrominance signal. The additional luminance signal ideally averages to zero, but in reality the signal averages to zero only if it is unchanging over time. Thus, the additional luminance signal will completely cancel only if it is unchanging. In signal processing terms, only common mode signals are completely canceled. Differential signals do not cancel each other out and will remain in the NTSC signal as a residual signal that may cause interference with the luminance signal. The amount of residual signal depends on the bandwidth of the additional luminance signal. The greater the bandwidth, the greater the amount of additional luminance signal that will feed through and become visible to the television viewer (in the form of interference).
Therefore, it can be appreciated that there is a significant need for a system and method for introducing an additional information signal into a video signal without the undesirable effects of signal interference or reducing bandwidth to avoid interference.